A device disclosed in the following patent document 1 is known as a conventional opening and closing device of this type. The opening and closing device thereof includes a piano frame, a keyboard lid (rotating body) rotatably attached to the piano frame, a device main body attached to the frame, a rotation arm having a base-end section rotatably attached to the device main body, a biasing spring (rotation-biasing means) which rotates and biases the rotation arm, presses the tip-end section thereof against the keyboard lid, and thereby rotates and biases the keyboard lid in the opening direction (first rotating direction), and a rotation damper (damper means) which minimizes the rotating speed of the rotation arm that rotates together with the keyboard lid when the keyboard lid rotates in the closing direction (second rotating direction), and thereby minimizes the rotating speed of the keyboard lid in the closing direction.
The rotation damper includes a casing provided at the device main body so as to be incapable of rotating, a rotating shaft rotatably provided at the casing thereof, and a damper mechanism provided between the rotating shaft and the casing, and the base-end section of the rotation arm is connected to the rotating shaft in a manner incapable of rotation. Accordingly, when the rotation arm rotates, the rotating shaft rotates, and the rotation thereof is suppressed to a low speed by the damper mechanism. As a result, the rotation of the rotation arm is suppressed to a low speed.